I am far from an expert, but my understanding is that memory for the PCI
devices is reserved in the 3 GB to 4GB range. I had this problem on our
server. 4GB of memory installed bit only 3.5GB reported. Enabling Memory
hole remapping showed the entire 4 GB.
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Hello,

We had a similar issue with new MBs for servers. Changing the Memory Hole
Remapping in the BIOS fixed the 3Gb limitation.